由 + agent: it's up to you, handled by
【五11】介词 / prepositions
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
由 introduces the person or group who takes charge of an action or makes a decision, and so hands them the task: 这道题由你来回答 ("you answer this question"), 这件事由班长负责 ("the class monitor is in charge of this"), 费用由公司承担 ("the company bears the cost"). The verb is often 负责, 决定, 提供, 承担, or a verb preceded by 来.
The order is: the task first, then 由 + agent. 由 is neutral and looks ahead to what is still to be done; it is more formal and more "organising" than 让 / 叫 (HSK 3), which get someone to act. It never says that someone suffers something: that is the job of 被 (HSK 3).
Don't confuse it with 由于 ("because of", HSK 3), which introduces a cause. Register: neutral, very common in instructions, rules and meetings.
Examples
这道题由你来回答吧。
Zhè dào tí yóu nǐ lái huídá ba.How about you answer this question?
今天的晚饭由我来做。
Jīntiān de wǎnfàn yóu wǒ lái zuò.I'll cook tonight's dinner.
这件事情由班长负责。
Zhè jiàn shìqing yóu bānzhǎng fùzé.The class monitor is in charge of this.
旅行的费用由公司承担。
Lǚxíng de fèiyong yóu gōngsī chéngdān.The company will cover the travel costs.
会议的时间由大家一起决定。
Huìyì de shíjiān yóu dàjiā yìqǐ juédìng.The time of the meeting will be decided by everyone together.
如果大家的意见不一样，最后由经理做决定。
Rúguǒ dàjiā de yìjiàn bù yíyàng, zuìhòu yóu jīnglǐ zuò juédìng.If opinions differ, the manager will make the final decision.
Common mistakes
✗ 我的钱包由小偷偷走了 → ✓ 我的钱包被小偷偷走了 (a misfortune takes 被, 由 only assigns a task).
✗ 我由回答这个问题 → ✓ 这个问题由我来回答 (the task first, then 由 + person + 来 + verb).
✗ 由于你来回答这个问题 → ✓ 由你来回答这个问题 (由于 means "because of", not "it's up to you").